The present invention relates to improvements in an electrode for use in electrical discharge surface treatment, arranged to cause discharge to take place between the electrode and a workpiece. The discharge energy is used to form a hard coating on the surface of the workpiece. Also disclosed are a manufacturing method therefor, a discharge surface treatment method and an apparatus therefor.
Hitherto, as a technique for coating the surface of a work to impart corrosion resistance and wear resistance to the surface, a discharge surface treatment method has been disclosed in, for example, Japanese Patent Unexamined Publication No. Hei. 5-148615. The foregoing technique uses an electrode in the form of a green compact composed of WC powder and Co powder so that a primary step (a depositing step) is performed. Then, a secondary step (a re-melting step) is performed after the electrode has been changed. The second electrode, may be, e.g., a copper electrode whose electrode wear is comparatively small. Thus, the foregoing method requires two steps to complete the treatment of the surface of a metal material. This conventional technique is excellent when it is used to form, on a steel material, a hard coating exhibiting satisfactory hardness and adhesiveness and having a thickness of tens of xcexcm. However, the method encounters difficulty when a hard coating having sufficient adhesiveness is to be formed on a sintered material, such as a hard alloy.
A discharge surface treatment method disclosed in Japanese Patent Unexamined Publication No. Hei. 9-192937, which is capable of forming a hard coating having sufficient adhesiveness on a hard alloy will now be described with reference to FIG. 16. Referring to FIG. 16, reference numeral 1 represents an electrode in the form of a green compact manufactured by compressing TiH2 powder, 2 represents a workpiece, 3 represents a working tank, 4 represents a working fluid and 5 represents a switching element for switching the voltage and current which are applied to the electrode 1 and the workpiece 2. Reference numeral 6 represents a control circuit for on/off controlling of the switching element 5. Reference numeral 7 represents a power source, 8 represents a resistor and 9 represents a formed hard coating. The discharge surface treatment performed with the foregoing structure enables a hard coating exhibiting excellent adhesiveness and having a thickness of several xcexcm to tens of xcexcm to be formed on the surface of steel or a hard alloy.
Each of the foregoing conventional techniques are characterized by using an electrode in the form of a green compact, and has an advantage in that components of the electrode can easily be melted due to the discharge energy, permitting a coating to easily be formed on the surface of the workpiece. However, the following three reasons have inhibited practical use of the foregoing method.
A first reason will now be described. The electrode in the form of the green compact is brittle and susceptible to damage. Therefore, machining to adapt the electrode to the shape of the workpiece, or machining to form screw holes for securing the electrode to the apparatus cannot easily be performed. Thus, the preparatory operations for the discharge surface treatment become too complicated, causing a substantial deterioration in the process efficiency. To overcome the above-mentioned problem, it might be feasible to sinter the electrode, in the form of the green compact, into a metal electrode for use. However, there arises a problem in that the processability of the sintered electrode deteriorates and a speed at which the hard coating can be formed is reduced.
A second reason will now be described. An electrode having a size satisfactory from the viewpoint of practical use cannot easily be formed. That is, an electrode arranged to be used in the surface treatment of a mold or the like and having a satisfactorily large size can be formed only when a high performance press is employed. Moreover, the fact that the pressure cannot be uniformly transmitted in the material when the powder material is compressed causes irregularities in the density to occur. Therefore, there arises a problem of, for example, cracks. Hence, it follows that the non-uniform hard coating formed on the workpiece results in a poorer quality product.
A third reason will now be described. A thick film cannot easily be formed. The conventional method cannot form a coating, the thickness of which is larger than several xcexcm to tens of xcexcm. A hard coating having a thickness larger than the above-mentioned value, required by industry, cannot be formed.
An elaboration will be given regarding the third reason above. Thin film formation has industrially been performed by physical evaporation, or chemical evaporation which is a dry process. A thick film cannot be formed by the foregoing method. Therefore, spray coating or the like must be employed at present. Spray coating methods capable of coating a variety of materials on a workpiece suffer from coarse texture of the formed coating. Therefore, spray coating cannot be applied in situations requiring precision and durability, such as forming a coating on a mold. The materials which can be spray coated are also excessively limited.
A conventional technique has been disclosed in Japanese Patent Unexamined Publication No. Hei. 8-300227 which relates to an electrode for discharge surface treatment and a method of surface treatment for a metal material. This method has the steps of using carbide, compressing it into an electrode and performing temporal sintering at a temperature lower than the sintering temperature so that an electrode is formed. The method is arranged to change the machining polarity after the discharge surface treatment has been performed to perform a process for further hardening the hard coating. Therefore, the temporal sintering process must be performed at a comparatively high temperature. This process demands that a temperature of 1100xc2x0 C. is maintained for 30 minutes. Since dense textures have been formed in the foregoing electrode, and manufactured by the temporal sintering process, secondary machining of the electrode cannot be easily performed. Unfortunately, the hard coating cannot be efficiently deposited on a workpiece, resulting in poor quality of the hard coating. When a dense hard coating is required, the machining operation must be performed for a long time. The foregoing method has another problem in that the depositing process is easily shifted to a profile-discharge process.
A method of manufacturing a mold will now be described as an example of the workpiece. The mold can be manufactured by any one of the following three methods. A first method is arranged such that a mold is subjected to heat treatment to impart required hardness and wear resistance. A second method uses surface modifying techniques to deposit a hard coating on a portion, or the overall surface of the mold, to prolong the mold""s lifetime. A third method uses a hard alloy or the like to manufacture the mold to maintain the accuracy of the mold for a long time. The third method is employed to manufacture a mold for automobiles or the like which are mass-produced, or to manufacture precise products.
In the present invention, a discharge surface treatment method is employed when a mold is the workpiece which must be processed according to the third method. According to the present invention, a discharge surface treatment method is provided for a mold which is interchangeable with or substitutable for a mold made of a hard alloy or a mold which partially uses the hard alloy. A conventional technique will now be described.
FIG. 17 shows an example of a mold for a die header which is used as a mold a precise forging process. A hard alloy block 101 is embedded in the central portion of a base metal 100 and machined by a profile discharge machine or a wire discharge machine to provide the actual mold surface. Additionally, a discharge surface treatment is performed to deposit a hard coating on the surface of the mold to increase the hardness of the surface which improves the durability of the mold. FIG. 17 shows the structure which is used when the discharge surface treatment is performed. A discharge surface treatment uses an electrode 103 in the form of a green compact to produce a hard coating on the surface of the mold having a thickness of about several xcexcm. Reference numeral 102 represents a shank for securing the electrode 103. As described above, the mold is manufactured by a plurality of steps including machining the base metal for the mold, embedding the hard alloy block, precise machining of the shape of the mold and discharge surface treatment for improving the surface of the mold.
The foregoing process for manufacturing the mold has two critical problems. A first problem arises due to the hard alloy block being force-fitted into the base material of the mold. Therefore, both of the base material of the mold and the hard alloy block must be machined with considerably excellent accuracy. Therefore, a long time and a great cost are required to manufacture the mold. A second problem is caused from a fact that the hard alloy block is made of a different material than the base material of the mold. As a result, the difference in the coefficient of thermal expansion causes cracks and breakage to easily occur. If the hard alloy block cannot be used due to breakage or cracks, the mold must be discarded or re-manufactured. Also, a long time and a great cost are required to manufacture the mold.
Therefore, a need for improvement is called for by a department which manufactures the molds and/or a department which uses the molds. However, an effective solution has not been provided.
Another case will now be described. In the automobile parts manufacturing industry, a mold for forging a connecting rod structured, for example, as shown in FIG. 18, is widely used. FIG. 19 shows a representative manufacturing process in the foregoing case. Recently, a high speed cutting technique has rapidly been improved. Therefore, a hard workpiece obtained by heat treatment can be subjected to a cutting operation. FIG. 20 shows results of a comparison of the time required to manufacture connecting rod molds between the high-speed cutting operation and the conventional discharge machining operation. As can be understood from FIG. 20, the high-speed cutting operation is more efficient than the conventional discharge machining operation.
Since the mold wears after it has been used as shown in 5 of FIG. 19, changing to a new mold or improving the accuracy of the worn mold is required. In a case of a large mold, as shown in FIG. 18, the hard alloy block cannot easily be embedded. A major portion of large molds of the foregoing type is usually made of die steel. Therefore, if the die-steel mold has been worn, heat treatment and surface improvement can only be partially performed to improve the durability. Therefore, the frequency of re-manufacturing the molds is raised excessively, causing the cost of manufacturing the mold to be dramatically increased.
The conventional method of forming a hard coating to a workpiece, such as a mold, by performing discharge surface treatment has been structured as described in Japanese Patent Unexamined Publication No. Hei. 5-148615.
The conventional method, however, suffers from thin thickness of the hard coating as shown in FIG. 21, deterioration in the characteristics of the material at high temperatures due to plastic deformation and insufficient tenacity. Therefore, it is difficult to use the mold having the hard coating formed thereon as a substitute for the hard alloy block. Therefore, the foregoing hard coating has been limitedly used to improve the surface of a hard alloy.
As described above, there arises a problem of increased time and manufacturing costs needed to manufacture the mold made of the hard alloy. In a case of a large mold into which the hard alloy block cannot be embedded, there arises a problem of increased frequency of re-manufacturing the molds and, therefore, the cost for manufacturing the mold cannot be reduced. The conventional method of forming the hard coating by the discharge surface treatment suffers from an unsatisfactorily small thickness. Therefore, the problems could not be previously overcome.
The present invention solves the above-mentioned problems experienced with the conventional techniques, and an object of the present invention is to obtain an electrode for discharge surface treatment which can easily be secondary-machined and which is free from reduction in a forming rate of a hard coating.
Another object of the present invention is to obtain an electrode for discharge surface treatment which is capable of forming, on a workpiece, a hard coating, which is capable of imparting special functions including lubricity, strength against high temperatures and wear resistance.
Another object of the present invention is to obtain an electrode for discharge surface treatment which is capable of forming a high-quality hard coating on a workpiece.
Another object of the present invention is to obtain a discharge surface treatment method which is capable of efficiently forming a hard coating on a workpiece, easily forming an electrode, forming a thick film of a hard coating in an arbitrary range of an area which is applicable to a variety of mechanical elements including molds, tools and mechanical parts.
Another object of the present invention is to obtain a discharge surface treatment method which is applied to a mold as a substitute for a mold made of a hard alloy, which exhibits a low cost, high accuracy and excellent durability, which can be quickly manufactured and which can be used repeatedly using only a simple repairing operation.
Since the present invention is structured as described above, the following effects can be obtained.
The electrode for discharge surface treatment according to the first invention attains an effect that it can easily be formed by a mechanical removing process, such as a turning operation, a grinding operation or a polishing operation or a discharging process. Moreover, a discharge surface treatment using the electrode can be performed such that a rate at which the hard coating which is formed on the work is formed is not reduced.
The electrode for discharge surface treatment according to the second invention attains an effect similar to that obtainable from the first invention and another effect that formability in the compression forming can significantly be improved.
The electrode for discharge surface treatment according to the third invention attain an effect similar to that obtainable from the first invention or the second invention.
The electrode for discharge surface treatment according to the fourth invention attains an effect similar to that obtainable from the first invention or the second invention and another effect that a hard coating capable of imparting special functions including lubricity, strength against high temperatures and wear resistance can be formed on a work by the discharge surface treatment using the electrode.
The electrode for discharge surface treatment according to the fifth invention attains an effect similar to that obtainable from the first invention or the second invention and another effect that a denser and high-quality hard coating free from irregularity of the hardness can be formed on a work by the discharge surface treatment using the electrode.
The method of manufacturing the electrode for discharge surface treatment according to the sixth invention attains an effect that an electrode for discharge surface treatment can be obtained which can easily be formed by a mechanical removing process, such as a turning operation, a grinding operation or a polishing operation or a discharging process and another effect that the discharge surface treatment using the electrode can be performed such that a rate at which the hard coating which is formed on the work is formed is not reduced.
The method of manufacturing the electrode for discharge surface treatment according to the seventh invention attains an effect similar to that obtainable from the sixth invention and another effect that the formability in the compression forming can significantly be improved.
The method of manufacturing the electrode for discharge surface treatment according to the eighth invention attains an effect similar to that obtainable from the sixth invention or the seventh invention.
The method of manufacturing the electrode for discharge surface treatment according to the ninth invention attains an effect similar to that obtainable from the sixth invention or the seventh invention. Moreover, another effect can be obtained in that a hard coating capable of imparting special functions including lubricity, strength against high temperatures and wear resistance can be formed on a work by the discharge surface treatment using the electrode.
The method of manufacturing the electrode for discharge surface treatment according to the tenth invention attains an effect similar to that obtainable from the sixth invention or the seventh invention. Moreover, another effect can be obtained in that a denser and high-quality hard coating free from irregularity of the hardness can be formed on a work by the discharge surface treatment using the electrode.
The discharge surface treatment method according to the eleventh and twelfth inventions attains an effect that the electrode for discharge surface treatment can easily be formed, a hard coating can efficiently be formed on a work and a discharge surface treatment method can be obtained which can be applied to a variety of mechanical parts including a mold, a tool and a mechanical element. Another effect can be obtained in that the masking process is not required because the hard coating can be formed in an area of the work which is substantially the same as the area of the electrode.
The discharge surface treatment method according to the thirteenth invention attains an effect similar to that obtainable from the eleventh invention and another effect that the structure can be simplified.
The discharge surface treatment method according to the fourteenth invention attains an effect similar to that obtainable from the eleventh invention. Another effect can be obtained in that machining can be performed while a small-size electrode is being scanned, a necessity for using a large-size and special-shape sintered electrode can be eliminated, the small-size electrode can be scanned on the overall curved surface of a work, such as a mold, having a three-dimensional free curved surface and a hard coating can be formed having the same thickness over the area of the work or such that the thickness is changed if necessary.
The discharge surface treatment method according to the fifteenth invention attains an effect similar to that obtainable from the eleventh invention. Another effect can be obtained in that a hard coating capable of imparting special functions including lubricity, strength against high temperatures and wear resistance can be formed on a work by the discharge surface treatment using the electrode.
The discharge surface treatment method according to the sixteenth invention attains an effect similar to that obtainable from the eleventh invention and another effect that a denser and high-quality hard coating free from irregularity of the hardness can be formed on a work by the discharge surface treatment using the electrode.
The discharge surface treatment method according to the seventeenth invention attains an effect similar to that obtainable from the eleventh invention and another effect that a mold coated with a hard coating which can be manufactured in a short time, the cost of which can be reduced and which exhibits satisfactory accuracy can be obtained. Another effect can be obtained in that a mold coated with a hard coating exhibiting excellent durability and repeated use of which is permitted with a simple modifying operation if the mold is worn.
The discharge surface treatment method according to the eighteenth invention attains an effect similar to that obtainable from the seventeenth invention. Another aspect can be obtained in that a mold coated with a hard coating exhibiting furthermore satisfactory durability can be obtained because a hard coating thicker than a hard coating formed in a portion of the mold in which the degree of wear is low is formed in a portion of the same in which the degree of wear is high.
The discharge surface treatment method according to the nineteenth invention attains an effect similar to that obtainable from the seventeenth invention. Another effect can be obtained in that a mold coated with a hard coating can be obtained with which re-manufacturing of the mold is not required, semipermanent use of the mold is permitted, costs required to manufacture the mold and maintain the same can considerably be saved and saving of energy and environmental friendliness are permitted because the amount of the material for manufacturing the mold can considerably be reduced.
The discharge surface treatment method according to the twentieth invention attains an effect similar to that obtainable from the nineteenth invention and another effect that modification of the mold can be completed in a considerably short time.
The discharge surface treatment apparatus according to the twenty-first and twenty-second inventions attains an effect that a discharge surface treatment apparatus can be obtained with which the electrode for discharge surface treatment can easily be formed, a hard coating can efficiently be formed on a work and adaptation to a variety of mechanical parts including a mold, a tool and a mechanical element is permitted. Another effect can be obtained in that the masking process is not required because the hard coating can be formed in an area of the work which is substantially the same as the area of the electrode.
The discharge surface treatment apparatus according to the twenty-third invention attains an effect similar to that obtainable from the twenty-first invention and another effect that the apparatus can be simplified.
The discharge surface treatment apparatus according to the twenty-fourth invention attains an effect similar to that obtainable from the twenty-first invention. Another effect can be obtained in that machining can be performed while a small-size electrode is being scanned, a necessity for using a large-size and special-shape sintered electrode can be eliminated, the small-size electrode can be scanned on the overall curved surface of a work, such as a mold, having a three-dimensional free curved surface and a hard coating can be formed having the same thickness over the area of the work or such that the thickness is changed if necessary.